Recording and reproduction of electrical impulses



0ct 5,'1937. 6.1.. DIMMICK 2,094,737

- RECORDING ANDREPRODUCTION OF ELECTRICAL IMPULSES Filed June 8, i953 2Sheets-sheaf; 1

INVENTOR GLEN/V LESL/f D/MM/C/f ATTORNEY Oct. 5, 1937. G, L, 1 M

RECRDING AND REPRODUCTION OF ELECTRICAL IMPULSES Filed June 8, 1933 2Sheets-Sheet 2 INVENTOR 6LAW 1554/: BY Lg ATTORNEY Patented Oct. 5, 1937PATENT OFFICE RECORDING AND REPRODUCTION OF ELECTRICAL IMPULSES GlennLeslie Dimmick, Westmont, N. J., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application June 8, 1933, Serial No.674,805

6 Claims.

This invention relates to the photographic recording and reproduction ofelectrical impulses, such as those of audio frequency, and has for itsprincipal object the provision of an improved apparatus and method ofoperation whereby the reproduced impulse distortion which tends toresult from the imperfect resolving power of the photographic record isminimized or avoided.

As pointed out in a copending application, Serial No. 610,302, filed May9, 1932 and assigned to the same assignee as the present application,the limited resolving power of the photographic film available forrecording audio or other impulses causes the valleys of the highfrequency waves to be filled in or fogged, thus partially rectifying thehigher frequency components of the recorded impulses. Also thehighfrequency peaks are likely to be underexposed and soft. The effecton the reproduced impulses is the introduction of a second harmonicdistortion and a distortion which results from. a change in the averagetransmission and follows the envelope of the higher frequencies. Inaccordance with this invention, both these types of distortion areavoided or minimized by the provision of two similar records displacedin phase by 180 degrees and arranged to be simultaneously reproduced andcombined for operating a sound reproducer or loud speaker.

The fogging of the valleys of the high frequencies in a film recordingresults in the following types of. distortion. Due to the loss oftransmission in the valleys the amplitude of the light modulation isdecreased and an attenuation'of the high frequencies results. The changein the shape of the wave would result in the introduction of both evenand odd harmonics. The reduction of the average transmission when highfrequencies are recorded results in extraneous sounds, the frequenciesof which depend upon the envelope of the high frequencies recorded. Thelatter distortion is by far the most serious since it producesdisagreeable extraneous sounds accompanying the sibilants. The method ofrecording and reproducing outlined, however, eliminates this zero shiftdistortion resulting from the change in the average transmission and atthe same time completely eliminates all even harmonic distortion. Oddharmonics and attenuation of the high frequencies are still present. Thefirst odd harmonic, the third, is not of such great importance becausethe filling in of the valleys of the recorded waves does not usuallybecome appreciable for frequencies below half of the frequency rangerecorded. For instance,

It will be apparent that the zero shift distortion and second harmonicdistortion, is eliminated by the push-pull recording and reproducingsystem if it is considered that the resulting electrical wave would besymmetrical about the time axis. It is obvious that the totaltransmission at the instant W--W shown on Figure 2 is the same as thetransmission at instant V-V which is displaced a half length along thetime axis. The electrical wave will therefore have the same numericalvalue at W-W and at V-V, but will have opposite signs. It can be provenmathematically that a wave which is symmetrical about the time axis isfree from even harmonic distortion and that the average value of thewave does not vary with its amplitude.

The invention will be better understood from the following descriptionwhen considered in connection with the accompanying drawings, and itsscope will be pointed out in the appended claims.

Referring to the drawings:

Fig. 1 is a diagrammatic illustration of a recording system to which theinvention has been applied,

Fig. 2 illustrates the type of record produced by the system of Fig. 1,

Fig. 3' is a diagrammatic showing of a system adapted to reproduce therecord of Fig. 2,

Fig. 4 illustrates a modified form of recording system which facilitatesthe elimination of ground or background noise,

Fig. 5 illustrates a light mask control element which forms a part ofthe system of Fig. 4, and

Fig. 6 illustrates the type of record which is produced by the system ofFig. 4 and is adapted to be reproduced by the system of Fig. 3.

The recording system of Fig. 1 includes a source 50 from which arectangularly-shaped light beam is applied through a lens II, a mask 52and a lens 13 to a galvanometer mirror M which is vibrated in accordancewith the impulses to be recorded. From the mirror M, the light beam isreflected through a lens IS, a light slit member l6 and lenses H and I8to aphotographic or light sensitive record which is moved lengthwise bymeans of a drum 20 and the cooperating rollers 2i and 22.

It will be observed that the light beam is biased to a position where itilluminates a rectangularlyshaped area midway between the ends of thelight slit in the member I6. When this beam is vibrated longitudinallyof the light slit, the two records A and B of. Fig. 2 are produced.These records are degrees out of phase with one another and, whenreproduced together, rectify the distortional elfects previouslymentioned.

The records A and B are reproduced by the apparatus of Fig. 3. Thisapparatus includes a source 23 from which light is projected through anoptical system 24, the records A and B on the film i9, a lens 25 andlenses 26 and 21 to a pushpull type of photo-electric or other lightsensitive device 28. The device 28 includes an anode 29 which isconnected through a battery 30 to an intermediate terminal of atransformer winding 3| and with cathodes 32 and 33 which are connectedrespectively to the opposite end terminals of the winding 3|.Inductively associated with the winding 3| is a secondary winding 34which is connected through suitable amplifying apparatus to a soundreproducer in a manner well known to those skilled in the art.

Since the records A and B are not only identical but are180 degrees outof phase, it necessarily follows that their simultaneous reproductionresults in the cancellation of the distortional effects previouslyresulting from fogging of the high frequency valleys and softening ofthe high frequency peaks. V V

The transparent section between the records A and B is of constant widthand of course leaves something to be desired where ground noise due tofilm grain, scratches, dust etc. is an important consideration. Therecording system of Fig. 4 is designed to eliminatesuch ground orbackground noise.

This apparatus differs from that of Fig. 1 in that therectangularly-apertured mask i2 is replaced by a triangularly-aperturedmask 35 which is arranged to be moved by an electric motor device 35energized in accordance with the envelope of the impulses to be recordedin any suitable manner such as that disclosed by United States PatentNo. 1,855,197.

As indicated more clearly in Fig. 5, the rectified or envelope currentreferred to in the aforesaid patent is supplied through a pair ofterminals to the cells 38 to 4! which are so wound on the pole pieces ofa magnet 42 as to cause the triangular light beam to move transverselyacross the light slit as indicated by the arrow C. This beam is alsomoved longitudinally of the light slit by the mirror i l which isvibrated in accordance with the impulses to be recorded. Under theseconditions, the records E and F (see Fig. 6) are not evenly spaced fromone another but the distance between them varies with the amplitude ofthe impulses, this result being effected by the movable mask 35 whichmoves the triangular light beam further across the light slit when theamplitude is high and vice versa. At all times, however, the spacing"between the records E and F is kept at the minimum permittingsatisfactory reproduction, thus maintaining a low ratio of ground tosignal noise. The records E and F may of 7 light source, means providedwith course be reproduced by the apparatus of Fig. 3.

Having thus described my invention, what I claim is:

1. An impulse recording system including a light source, means providedwith a triangular aperture for defining the shape of the beam of lightfrom said source, a member provided with a narrow slit, means forimaging the said aperture on the said slit, means for vibrating the saidbeam in accordance with impulses to be recorded and means for moving thesaid aperture in accordance with the envelope of said impulses.

2. An impulse recording system including a light source, means providedwith a triangular aperture for defining the shape of the beam of lightfrom said source, a member provided with a narrow slit, means forforming an image of said slit on a film, means for imaging the saidaperture on the said slit, means for vibrating the said beam inaccordance with impulses to be recorded and means for moving thesaid'aperture in accordance with the envelope of said impulses.

3. An impulse recording system including a light source, means providedwith an aperture of varying dimensions for defining the shape of thebeam oflight from said source, a member provided with a narrow slit,means for imaging the said aperture on the said slit, means forvibrating the said beam in accordance with impulses to be recorded andmeans for moving the said aperture in accordance with the envelope ofsaid impulses, the variation in dimensions of said aperture beingtransverse of the said slit.

4. An' impulse recording system including a light source, means providedwith a triangular aperture for defining the shape of the beam of lightfrom saidsource, a'member provided with a narrow slit, means for imagingthe said aperture on the said slit, means for vibrating the said beamlongitudinally of the said slit in accordance with impulses to berecorded and means for mov ing the said aperture in accordance with theenvelope of said impulses.

5. An impulse recording system including a a triangular aperture fordefining the shape of the beam of light from said source, a memberprovided with a narrow slit, means for imaging the said aperture on thesaid slit, means for vibrating the said beam longitudinally of said slitin accordance with impulses to be recorded and means for moving the saidaperture transversely of the slit in accordance with the envelope ofsaid impulses.

6. An impulse recording system including a light source, means providedwith a triangular aperture for defining the shape of the beam of lightfrom said source, a member provided with a narrow slit, means forforming an image of said slit on a film, means for imaging the saidaperture on the said slit, means for vibrating the said beamlongitudinally of said slit in accordance with impulses to be recordedand means for moving the said aperture transversely of the slit inaccordance with the envelope of said impulses.

GLENN LESLIE DIMMICK.

